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Absorbers

Absorption Coefficient

The factor by which photons are absorbed as they travel a unit distance through a material.

Active solar heater

A solar water or space-heating system that moves heated air or water using pumps or fans.

AGM

Absorbed Glass Mat, a newer type of battery construction that uses saturated absorbent glass mats rather than gelled or liquid electrolyte. Somewhat more expensive than flooded (liquid), but offers very good reliability.

Amorphous semiconductor

A noncrystalline semiconductor material. Easier and cheaper to make than crystalline, but less efficient and slowly degrades over time. Also called thin film.

Ampere Hour (Ah)

The quantity of electrical energy equal to the flow of current of one ampere for one hour. The term is used to quantify the energy stored in a battery.

Angle of incidence

Angle between the normal to a surface and the direction of incident radiation; applies to the aperture plane of a solar collector. Most modern solar panels have only minor reductions in power output within plus/minus 15 degrees. The loss is a function of the cosine, so at 45 degree angle, output drops off by about 30%.

Anode

The positive electrode in an electrochemical cell (battery). Also, the earth ground in a cathodic protection system. Also, the positive terminal of a diode.

Antireflection Coating

A thin coating of a material, which reduces the light reflection and increases light transmission, applied to a photovoltaic cell surface.

Array

A collection of electrically connected photovoltaic (PV) modules.

Array Current

The electrical current produced by a PV array when it is exposed to sunlight.

autonomous system

A stand-alone PV system that has no back-up generating source. May or may not include storage batteries. Most battery systems are designed for a certain minimum "days of autonomy" - which means that the batteries can supply sufficient power with no sunlight to charge the batteries. This varies from 3-5 days in the sunbelt, to 5 to 10 days elsewhere.

Azimuth

Angle between the north direction and the projection of the surface normal into the horizontal

Balance of system (BOS)

Represents all components and costs other than the PV modules. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, batteries, indirect storage, and related costs.

Base Load

The average amount of electric power that a utility must supply in any period.

Blocking Diode

A diode used to restrict or block reverse current from flowing backward through a module. Alternatively, diode connected in series to a PV string; it protects its modules from a reverse power flow and, thus, against the risk of thermal destruction of solar cells.

blocking diode

A diode used to restrict or block reverse current from flowing backward through a module. [UL 1703] Alternatively, diode connected in series to a PV string; it protects its modules from a reverse power flow and, thus, against the risk of thermal destruction of solar cells.

Bypass diode

A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. [UL 1703] Alternatively, diode connected anti-parallel across a part of the solar cells of a PV module. It protects these solar cells from thermal destruction in case of total or partial shading, broken cells, or cell string failures of individual solar cells while other cells are exposed to full light.

Cadmium Telluride (CdTe)

A polycrystalline thin-film photovoltaic material.

Capacity

The total number of ampere-hours that can be withdrawn from a fully charged battery at a specified discharge rate and temperature.

Cathode

The negative electrode in an electrochemical cell. Also, the negative terminal of a diode.

Cathodic Protection

A method of preventing oxidation (rusting) of exposed metal structures, such as bridges and pipelines, by imposing between the structure and the ground a small electrical voltage that opposes the flow of electrons and that is greater than the voltage present during oxidation.

cell

The basic unit of a photovoltaic panel or battery

cell barrier

A very thin region of static electric charge along the interface of the positive and negative layers in a photovoltaic cell. The barrier inhibits the movement of electrons from one layer to the other, so that higher-energy electrons from one side diffuse preferentially through it in one direction, creating a current and thus a voltage across the cell. Also called depletion zone, cell junction, or space charge.

cell junction

The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone.

charge controller

An electronic device which regulates the voltage applied to the battery system from the PV array. Essential for ensuring that batteries obtain maximum state of charge and longest life.

Chemical Vapour Deposition

A method of depositing thin semiconductor films. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.

CIS

Copper-Indium-Diselelide.

combined collector

A photovoltaic device or module that provides useful heat energy in addition to electricity.

concentrator

A PV module that uses optical elements to increase the amount of sunlight incident on a PV cell. Concentrating arrays must track the sun and use only the direct sunlight because the diffuse portion cannot be focused onto the PV cells. Efficiency is increased, but lifespan is usually decreased due to the high heat.

conversion efficiency (cell or module)

The ratio of the electric energy produced by a photovoltaic device (under one-sun conditions) to the energy from sunlight incident upon the cell.

Converter

A unit that converts a dc voltage to another dc voltage.

Crystalline Silicon

A type of PV cell made from a single crystal or polycrystalline slice of silicon.

Cutoff Voltage

The voltage levels (activation) at which the charge controller disconnects the array from the battery or the load from the battery.

cycle life

Number of discharge-charge cycles that a battery can tolerate under specified conditions before it fails to meet specified criteria as to performance (e.g., capacity decreases to 80-percent of the nominal capacity).

Depth of Discharge

The percent of the rated battery capacity that has been withdrawn.

diffuse insolation

Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.

Diffuse Radiation

Radiation received from the sun after reflection and scattering by the atmosphere and ground.

Direct Beam Radiation

Radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.

direct current(dc)

Electric current in which electrons flow in one direction only. Opposite of alternating current.

direct insolation

Sunlight falling directly upon a collector. Opposite of diffuse insolation.

discharge rate

The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.

distributed systems

Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system.

DOD

'Depth of Discharge,' from 100-percent state of charge (SOC), in a battery or battery system.

Duty Cycle

The ratio of active time to total time. Used to describe the operating regime of appliances or loads in PV systems.

DVM

digital Volt-meter.

Efficiency

The ratio of output power (or energy) to input power (or energy). Expressed in percent.

EFG

Edge definde Film Growth, A method for making sheets of polycrystalline silicon in which molten silicon is drawn upward by capillary action through a mold.

electric circuit

Path followed by electrons from a power source (generator or battery) through an external line (including devices that use the electricity) and returning through another line to the source.

Electrical Grid

An integrated system of electricity distribution, usually covering a large area.

Electrolyte

The medium that provides the ion transport mechanism between the positive and negative electrodes of a battery.

energy as was required in its manufacture. For solar electric panels, this is about 16-20 months.

energy density

The ratio of energy available from a battery to its volume (Wh/1) or mass (Wh/kg). "watts to weight" ratio.

energy payback time

The time required for any energy producing system or device to produce as much

Equalisation Charge

The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short time.

EVA

Ethylene-Vinile-Acetate Foil, it will be used by module production for covering the cells.

Fill Factor (FF)

For an I-V curve, the ratio of the maximum power to the product of the open-circuit voltage and the short-circuit current. Fill factor is a measure of the "squareness" of the I-V curve.

Fixed Tild Array

A PV array set in at a fixed angle with respect to horizontal.

flat-plate PV

Refers to a PV array or module that consists of nonconcentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun-angles in relation to the array.

float charge

Float charge is the voltage required to counteract the self-discharge of the battery at a certain temperature.

float life

Number of years that a battery can keep its stated capacity when it is kept at float charge (see float charge).

Frequency

The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).

fuel cell

A device that converts the energy of a fuel directly to electricity and heat, without combustion. Because there is no combustion, fuel cells give off few emissions; because there are no moving parts, fuel cells are quiet.

Gallium Arsenide (GaAs)

Gassing

Gel Type Battery

Lead-acid battery in which the electrolyte is composed of a silica gel matrix.

Grid

Term used to describe an electrical utility distribution network.

Grid Connected PV System

A PV system in which the PV array acts like a central generating plant, supplying power to the grid.

ground loop

An undesirable feedback condition caused by two or more circuits sharing a common electrical line, usually a grounded conductor.

hot spot

An undesirable phenomenon of PV device operation whereby one or more cells within a PV

Hybrid System

A PV system that includes other sources of electricity generation, such as wind or diesel generators.

I-U Characteristics

The plot of the current versus voltage characteristics of a photovoltaic cell, module, or array. Three important points on the I-V curve are the open-circuit voltage, short-circuit current, and peak power operating point.

I-V curve

A graphical presentation of the current versus the voltage from a photovoltaic device as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterized cell performance.

I-V data

The relationship between current and voltage of a photovoltaic device in the power-producing quadrant, as a set of ordered pairs of current and voltage readings in a table, or as a curve plotted in a suitable coordinate system (i.e., Cartesian). [ASTM E 1036]

insolation

Sunlight, direct or diffuse; from 'incident solar radiation.' Not to be confused with 'insulation.' Equal to about 1000 watts per square meter at high noon in Dodge City.

interconnect

A conductor within a module or other means of connection which provides an electrical interconnection between the solar cells. [UL 1703]

Inverter

(Power Conditioning Unit, PCU, or Power Conditioning System, PCS) In a PV system, an inverter converts dc power from the PV array/battery to ac power compatible with the utility and ac loads.

Irradiance

The solar power incident on a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals Insolation.

Irradiation

The solar radiation incident on an area over time. Equivalent to energy and usually expressed in kilowatt-hours per square meter.

Joule (J)

Unit of energy equal to 1/3600 kilowatt-hours.

Junction Box

A PV generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

junction diode

A semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes.

Kilowatt (kW)

One thousand watts. A unit of power.

Kilowatt Hour (kWh)

One thousand watt-hours. A unit of energy. Power multiplied by time equals energy.

light trapping

The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers.

light-induced defects

Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.

line-commutated inverter

An inverter that is tied to a power grid or line. The commutation of power (conversion from dc to ac) is controlled by the power line, so that, if there is a failure in the power grid, the PV system cannot feed power into the line.

Load

The amount of electric power used by any electrical unit or appliance at any given time.

Load Current

The current required by the electrical device.

Low Voltage Cutoff

The voltage level at which a controller will disconnect the load from the battery.

Maintanace Free Battery

A sealed battery to which water cannot be added to maintain electrolyte level.

maximum power point (MPP)

The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. [UL 1703] For a typical silicon cell panel, this is about 17 volts for a 36 cell configuration.

maximum power point tracker (MPPT)

A power conditioning unit that automatically operates the PV-generator at its maximum power point under all conditions. An MPPT will typically increase power delivered to the system by 10% to 40%, depending on climate conditions and battery state of charge. You usually get more gain in winter and in colder weather due to the higher panel output.Most MPPT controllers are down converters - from a higher voltage to a lower one.

microgroove

A small groove scribed into the surface of a cell which is filled with metal for contacts.

MPP

Maximum Power Point; The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 V.

MPPT

Maximum Power point Tracker; Means of a power conditioning unit that automatically operates the PV-generator at its MPP under all conditions.

multicrystalline

Material that is solidified at such as rate that many small crystals (crystallites) form. The atoms within a single crystallite are symmetrically arranged, whereas crystallites are jumbled together. These numerous grain boundaries reduce the device efficiency. A material composed of variously oriented, small individual crystals. (Sometimes referred to as polycrystalline or semicrystalline).

multijunction device

A photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum, to achieve greater overall efficiency.

NOCT

Nominal Operating Cell Temperature. The solar cell temperature at a reference environment defined as 800 W/m2 irradiance, 20°C ambient air temperature, and 1 m/s wind speed with the cell or module in an electrically open circuit state.

one-axis tracking

A system capable of rotating about one axis, usually following the sun from East to West.

Open Cirquit Voltage

The maximum voltage produced by an illuminated photovoltaic cell, module, or array with no load connected. This value will increase as the temperature of the PV material decreases.

Operating Point

The current and voltage that a module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.

Overcharge

Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.

Panel

A designation for a number of PV modules assembled in a single mechanical frame.

parallel connection

A way of joining two or more electricity-producing devices (i.e., PV cells or modules) by connecting positive leads together and negative leads together; such a configuration increases the current.

passive solar home

A house that uses a room or another part of the building as a solar collector, as opposed to active solar, such as PV.

Peak Load

The maximum load demand on a system.

peak load; peak demand

The maximum load, or usage, of electrical power occurring in a given period of time, typically a day.

Peak Sun Hours

The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 W/m2.

Peak Watt (Wp)

The amount of power a photovoltaic module will produce at standard test conditions (normally 1000 W/m2 and 25° cell temperature).

Photon -A particle of light that acts as an individual unit of energy. Its energy depends on wavelenght.

photovoltaic (PV)

Pertaining to the direct conversion of light into electricity.

photovoltaic (PV) array

An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module.

photovoltaic (PV) cell

The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (dc voltage and current).

photovoltaic (PV) efficiency

The ratio of electric power produced by a cell at any instant to the power of the sunlight striking the cell. This is typically about 9% to 14% for commercially available cells.

photovoltaic (PV) generator

The total of all PV strings of a PV power supply system, which are electrically interconnected.

photovoltaic (PV) module

The smallest environmentally protected, essentially planar assembly of solar cells and ancillary parts, such as interconnections, terminals, [and protective devices such as diodes] intended to generate DC power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate). [UL 1703]

photovoltaic (PV) panel

often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current).

photovoltaic (PV) system

A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.

Polycrystalline Silicon

A material used to make PV cells which consist of many crystals as contrasted with single crystal silicon.

power conditioning equipment

Electrical equipment, or power electronics, used to convert power from a photovoltaic array into a form suitable for subsequent use. A collective term for inverter, converter, battery charge regulator, and blocking diode.

Power Factor

The cosine of the phase angle between the voltage and the current waveforms in an ac circuit. Used as a designator for inverter performance. A power factor of 1 indicates current and voltage are in phase and power is equal to the product of volt-amperes. (no reactive power).

Primary Battery

A battery whose initial capacity cannot be restored by charging.

Pulse Width Modulated (PWM)

PWM inverters are the most expensive, but produce a high quality of output signal at minimum current harmonics. The output voltage is very close to sinusoidal.

PV

Abbreviation for photovoltaic(s).

Pyranometer

An instrument used for measuring global solar irradiance.

Pyrheliometer

An instrument used for measuring direct beam solar irradiance. Uses an aperture of 5.7° to transcribe the solar disc.

Rated Module Current

The current output of a PV module measured at standard test conditions of 1,000 w/m2 and 25°C cell temperature.

Reactive Power

The sine of the phase angle between the current and voltage waveforms in an AC system.

rectifier

A device that converts ac to dc, as in a battery charger or converter. See inverter and diode.

Remote Site

A site not serviced by an electrical utility grid.

remote systems

Systems off of the utility grid. Out in the boondocks.

resistive voltage drop

The voltage developed across a cell by the current flow through the resistance of the cell.

reverse bias

Condition where the current producing capability of a PV cell is significantly less than that of other cells in its series string. This can occur when a cell is shaded, cracked, or otherwise degraded or when it is electrically poorly matched with other cells in its string.

self discharge

The rate at which a battery, without a load, will lose its charge. This can vary considerably depending on the type of battery and age. It can be as low as 3% a month for a new AGM battery, and as high as 10% a week for an older Lead-Antimony (industrial) battery.

Self Distance

The rate at which a battery, without a load, will lose its charge.

semicrystalline

See 'Multicrystalline.'

series connection

A way of joining photovoltaic cells or batteries by connecting positive leads to negative leads; such a configuration increases the voltage.

shelf life of batteries

The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

Short Cirquit Current

The current produced by an illuminated PV cell, module, or array when its output terminals are shorted.

short-circuit current (Isc)

The current flowing freely from a photovoltaic cell through an external circuit that has no load or resistance; the maximum current possible.

shunt regulator

Type of a battery charge regulator where the charging current is controlled by a switch or transistor connected in parallel with the PV panel. Overcharging of the battery is prevented by shorting the PV output. Shunt regulators are common in PV systems as they are relatively cheap to build and simple to design. Series regulators usually have better control and charge characteristics. Most newer controllers have gone to series regulation.

sine wave inverter

An inverter that produces utility-quality, sine wave power forms.

Single Crystall Silicon

Material with a single crystalline formation. Many PV cells are made from single crystal silicon.

single-crystal material

A material that is composed of a single crystal or a few large crystals.

solar constant

The strength of sunlight; 1353 watts per square meter in space and about 1000 watts per square meter at sea level at the equator at solar noon. It increases at higher altitudes.

solar energy

Energy from the sun. The heat that builds up in your car when it is parked in the sun is an example of solar energy.

solar noon

That moment of the day that divides the daylight hours for that day exactly in half. To determine solar noon, calculate the length of the day from the time of sunset and sunrise and divide by two. Solar noon may be quite a bit different from 'clock' noon.

solar spectrum

The total distribution of electromagnetic radiation emanating from the sun.

solar thermal electric

Method of producing electricity from solar energy by using focused sunlight to heat a working fluid, which in turn drives a turbogenerator.

solar-grade silicon

Intermediate-grade silicon used in the manufacture of solar cells. Less expensive than electronic-grade silicon.

split-spectrum cell

A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity. See 'mulitjunction device.'

square wave inverter

The inverter consists of a dc source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence, at a certain frequency. The square wave inverter is the simplest and the least expensive to purchase, but it produces the lowest quality of power.

stand-alone (PV system)

An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage.

stand-off mounting

Technique for mounting a PV array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle.

state of charge (SOC)

The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

String

A number of modules or panels interconnected electrically in series to produce the operating voltage required by the load.

substrate

The physical material upon which a photovoltaic cell is made.

Subsystem

Any one of several components in a PV system (i.e., array, controller, batteries, inverter, load).

sulfation

A condition that afflicts unused and discharged batteries; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge.

Sun Path Diagram

Graphical representation of the Sun's height and azimuth

superstrate

The covering on the sun side of a PV module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum.

System Operating Voltage

The array output voltage under load. The system operating voltage is dependent on the load or batteries connected to the output terminals.

thermal electric

Electric energy derived from heat energy, usually by heating a working fluid, which drives a turbogenerator. See 'solar thermal electric.'

thermophotovoltaic (TPV) device

A device that converts secondary thermal radiation, re-emitted by an absorber or heat source, into electricity; The device is designed for maximum efficiency at the wavelength of the secondary radiation.

thick-crystalline materials

Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons.

Thin Film PV Module

A PV module constructed with sequential layers of thin film semiconductor materials.

Tilt Angle

The angle of inclination of a solar collector measured from the horizontal.

total internal reflection

The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must eventually be absorbed by the semiconductor.

tracking array

PV array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy. Typically, a single axis tracker will give you 15% to 25% more power per day, and dual axis tracking will add about 5% to that. Depends somewhat on latitude and season.

trickle charge

A charge at a low rate, balancing through self-discharge losses, to maintain a cell or battery in a fully charged condition.

two-axis tracking

A system capable of rotating independently about two axes (e.g., vertical and horizontal) and following the sun for maximum efficiency of the solar array.

Uninterruptible Power Supply (UPS)

The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

utility-interactive inverter

An inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the PV system's output is fully synchronized with the utility power.

VAC

Volts ac

Varistor

A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

Vented Cell

A battery designed with a vent mechanism to expel gases generated during charging.

Vmp

Voltage at maximum power

Voc

Open-circuit voltage

Voltage at Maximum Power

The voltage at which maximum power is available from a module.

waveform

The shape of the curve graphically representing the change in the ac signal voltage and current amplitude, with respect to time.

Zenith Angle

The angle between directly overhead and the line intersecting the sun. (90°- zenith) is the elevation angle of the sun above the horizon.

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